Airbnb Javascript 编码规范() {

使用更加合适的方适编写 JavaScript

其它规范

类型 Types
引用 References
对象 Objects
数组 Arrays
解构 Destructuring
字符串 Strings
函数 Functions
箭头函数 Arrow Functions
类 & 构造函数 Classes & Constructors
模块 Modules
迭代器和Generators Iterators and Generators
属性 Properties
变量 Variables
提升 Hoisting
比较运算符 & 等号 Comparison Operators & Equality
块作用域 Blocks
Control Statements
注释 Comments
空白 Whitespace
逗号 Commas
分号 Semicolons
类型转换 Type Casting & Coercion
命名规范 Naming Conventions
存取器 Accessors
事件 Events
jQuery
ECMAScript 5 兼容性
ECMAScript 6+ (ES 2015+) 规范
测试 Testing
性能 Performance
资源 Resources
谁在使用
Translation
The JavaScript Style Guide Guide
Chat With Us About JavaScript
Contributors
License

类型 Types

1.1 基本类型: 直接使用.

string
number
boolean
null
undefined

const foo = 1;
let bar = foo;

bar = 9;

console.log(foo, bar); // => 1, 9

1.2 复杂类型: 使用引用.

object
array
function

const foo = [1, 2];
const bar = foo;

bar[0] = 9;

console.log(foo[0], bar[0]); // => 9, 9

⬆ 返回目录

引用 References

2.1 对所有引用类型使用 const,不要使用 var。 eslint: prefer-const, no-const-assign

为什么？这能保证无法再对引用进行赋值，避免产生 bugs 和无法理解的代码。
```
// bad
var a = 1;
var b = 2;

// good
const a = 1;
const b = 2;
```

2.2 如果你必须重新给引用赋值，请使用 let 代替 var。 eslint: no-var jscs: disallowVar

为什么？let 是块级作用域，var 是函数作用域。
```
// bad
var count = 1;
if (true) {
  count += 1;
}

// good, use the let.
let count = 1;
if (true) {
  count += 1;
}
```

2.3 let 和 const都是块级作用域。

// const and let only exist in the blocks they are defined in.
{
  let a = 1;
  const b = 1;
}
console.log(a); // ReferenceError
console.log(b); // ReferenceError

⬆ 返回目录

对象 Objects

3.1 使用字面量定义 object。 eslint: no-new-object

// bad
const item = new Object();

// good
const item = {};

3.2 使用计算属性（表达式）命名对象的动态属性。

为什么？这让你可以在一个地方定义对象的所有的属性。

function getKey(k) {
  return `a key named ${k}`;
}

// bad
const obj = {
  id: 5,
  name: 'San Francisco',
};
obj[getKey('enabled')] = true;

// good
const obj = {
  id: 5,
  name: 'San Francisco',
  [getKey('enabled')]: true,
};

3.3 使用方法简写。 eslint: object-shorthand jscs: requireEnhancedObjectLiterals

// bad
const atom = {
  value: 1,

  addValue: function (value) {
    return atom.value + value;
  },
};

// good
const atom = {
  value: 1,

  addValue(value) {
    return atom.value + value;
  },
};

3.4 使用属性简写。 eslint: object-shorthand jscs: requireEnhancedObjectLiterals

为什么？这样可以更简洁。

const lukeSkywalker = 'Luke Skywalker';

// bad
const obj = {
  lukeSkywalker: lukeSkywalker,
};

// good
const obj = {
  lukeSkywalker,
};

3.5 简写的属性放在所有属性的开始位置。

为什么？这样可以清楚的知道哪个属性使用的简写。

const anakinSkywalker = 'Anakin Skywalker';
const lukeSkywalker = 'Luke Skywalker';

// bad
const obj = {
  episodeOne: 1,
  twoJediWalkIntoACantina: 2,
  lukeSkywalker,
  episodeThree: 3,
  mayTheFourth: 4,
  anakinSkywalker,
};

// good
const obj = {
  lukeSkywalker,
  anakinSkywalker,
  episodeOne: 1,
  twoJediWalkIntoACantina: 2,
  episodeThree: 3,
  mayTheFourth: 4,
};

3.6 只有在属性名不合法时使用引号。 eslint: quote-props jscs: disallowQuotedKeysInObjects

Why? In general we consider it subjectively easier to read. It improves syntax highlighting, and is also more easily optimized by many JS engines.
```
// bad
const bad = {
  'foo': 3,
  'bar': 4,
  'data-blah': 5,
};

// good
const good = {
  foo: 3,
  bar: 4,
  'data-blah': 5,
};
```

3.7 不要直接调用 Object.prototype，hasOwnProperty，propertyIsEnumerable，isPrototypeOf。

为什么？这些方法可能是对象的属性 { hasOwnProperty: false }，或者对象是 null (Object.create(null))。

// bad
console.log(object.hasOwnProperty(key));

// good
console.log(Object.prototype.hasOwnProperty.call(object, key));

// best
const has = Object.prototype.hasOwnProperty; // cache the lookup once, in module scope.
/* or */
import has from 'has';
// ...
console.log(has.call(object, key));

3.8 首选对象展开运算符 Object.assign to shallow-copy objects. Use the object rest operator to get a new object with certain properties omitted.

// very bad
const original = { a: 1, b: 2 };
const copy = Object.assign(original, { c: 3 }); // this mutates `original` ಠ_ಠ
delete copy.a; // so does this

// bad
const original = { a: 1, b: 2 };
const copy = Object.assign({}, original, { c: 3 }); // copy => { a: 1, b: 2, c: 3 }

// good
const original = { a: 1, b: 2 };
const copy = { ...original, c: 3 }; // copy => { a: 1, b: 2, c: 3 }

const { a, ...noA } = copy; // noA => { b: 2, c: 3 }

⬆ 返回目录

数组 Arrays

4.1 通过字面量创建数组。eslint: no-array-constructor

// bad
const items = new Array();

// good
const items = [];

4.2 使用 Array#push 而不是通过下标赋值。

const someStack = [];

// bad
someStack[someStack.length] = 'abracadabra';

// good
someStack.push('abracadabra');

4.3 使用展开运算符 ... 来复制数组.

// bad
const len = items.length;
const itemsCopy = [];
let i;

for (i = 0; i < len; i += 1) {
  itemsCopy[i] = items[i];
}

// good
const itemsCopy = [...items];

4.4 使用 Array.from 来转换类数组对象.

const foo = document.querySelectorAll('.foo');
const nodes = Array.from(foo);

4.5 在数组回调方法中使用 return。函数声明只有一行时可以省略 return。 8.2. eslint: array-callback-return

// good
[1, 2, 3].map((x) => {
  const y = x + 1;
  return x * y;
});

// good
[1, 2, 3].map(x => x + 1);

// bad
const flat = {};
[[0, 1], [2, 3], [4, 5]].reduce((memo, item, index) => {
  const flatten = memo.concat(item);
  flat[index] = flatten;
});

// good
const flat = {};
[[0, 1], [2, 3], [4, 5]].reduce((memo, item, index) => {
  const flatten = memo.concat(item);
  flat[index] = flatten;
  return flatten;
});

// bad
inbox.filter((msg) => {
  const { subject, author } = msg;
  if (subject === 'Mockingbird') {
    return author === 'Harper Lee';
  } else {
    return false;
  }
});

// good
inbox.filter((msg) => {
  const { subject, author } = msg;
  if (subject === 'Mockingbird') {
    return author === 'Harper Lee';
  }

  return false;
});

⬆ 返回目录

4.6 Use line breaks after open and before close array brackets if an array has multiple lines

// bad
const arr = [
  [0, 1], [2, 3], [4, 5],
];

const objectInArray = [{
  id: 1,
}, {
  id: 2,
}];

const numberInArray = [
  1, 2,
];

// good
const arr = [[0, 1], [2, 3], [4, 5]];

const objectInArray = [
  {
    id: 1,
  },
  {
    id: 2,
  },
];

const numberInArray = [
  1,
  2,
];

⬆ back to top

解构赋值 Destructuring

5.1 当对象有多个属性时使用解构赋值。 jscs: requireObjectDestructuring

为什么? 解构可以避免为这些属性创建临时引用。

// bad
function getFullName(user) {
  const firstName = user.firstName;
  const lastName = user.lastName;

  return `${firstName} ${lastName}`;
}

// good
function getFullName(user) {
  const { firstName, lastName } = user;
  return `${firstName} ${lastName}`;
}

// best
function getFullName({ firstName, lastName }) {
  return `${firstName} ${lastName}`;
}

5.2 数组使用解构。 jscs: requireArrayDestructuring

const arr = [1, 2, 3, 4];

// bad
const first = arr[0];
const second = arr[1];

// good
const [first, second] = arr;

5.3 返回多个值时使用对象解构，而不是数组解构。 jscs: disallowArrayDestructuringReturn

Why? You can add new properties over time or change the order of things without breaking call sites.

// bad
function processInput(input) {
  // then a miracle occurs
  return [left, right, top, bottom];
}

// the caller needs to think about the order of return data
const [left, __, top] = processInput(input);

// good
function processInput(input) {
  // then a miracle occurs
  return { left, right, top, bottom };
}

// the caller selects only the data they need
const { left, top } = processInput(input);

⬆ 返回目录

字符串 Strings

6.1 对字符串使用单引号 ''。 eslint: quotes jscs: validateQuoteMarks

// bad
const name = "Capt. Janeway";

// bad - template literals should contain interpolation or newlines
const name = `Capt. Janeway`;

// good
const name = 'Capt. Janeway';

6.2 Strings 的长度超过100个字符时不要使用加号写成多行。

Why? Broken strings are painful to work with and make code less searchable. 为什么？被破坏的 Strings 读起来是痛苦的，使用代码更少易搜索。

// bad
const errorMessage = 'This is a super long error that was thrown because \
of Batman. When you stop to think about how Batman had anything to do \
with this, you would get nowhere \
fast.';

// bad
const errorMessage = 'This is a super long error that was thrown because ' +
  'of Batman. When you stop to think about how Batman had anything to do ' +
  'with this, you would get nowhere fast.';

// good
const errorMessage = 'This is a super long error that was thrown because of Batman. When you stop to think about how Batman had anything to do with this, you would get nowhere fast.';

6.3 什么程序构造 Strings 时，使用字符串模版。 eslint: prefer-template template-curly-spacing jscs: requireTemplateStrings

Why? Template strings give you a readable, concise syntax with proper newlines and string interpolation features.

// bad
function sayHi(name) {
  return 'How are you, ' + name + '?';
}

// bad
function sayHi(name) {
  return ['How are you, ', name, '?'].join();
}

// bad
function sayHi(name) {
  return `How are you, ${ name }?`;
}

// good
function sayHi(name) {
  return `How are you, ${name}?`;
}

6.4 禁止在 string 上使用 eval()，这将会产生漏洞。eslint: no-eval

6.5 不要在不必要的地方使用转意。 eslint: no-useless-escape

Why? Backslashes harm readability, thus they should only be present when necessary. 为什么？反斜杠有损可读性。
```
// bad
const foo = '\'this\' \i\s \"quoted\"';

// good
const foo = '\'this\' is "quoted"';
const foo = `my name is '${name}'`;
```

⬆ 返回目录

函数 Functions

7.1 使用函数表达式代替函数声明。 eslint: func-style jscs: disallowFunctionDeclarations

为什么？函数声明具有提升作用，这意味着可以在文件中定义前就可以引用（使用），这有损可读性和可维护性。好果你发现函数定义太庞大太复杂，影响了你对文件中其它内容的理解，这意味着你应该将它提取成模块！不要忘记命名函数，匿明函数在产生 bug 时很难定位。
```
// bad
function foo() {
  // ...
}

// bad
const foo = function () {
  // ...
};

// good
const foo = function bar() {
  // ...
};
```

7.2 使用括号包裹立即执行函数。 eslint: wrap-iife jscs: requireParenthesesAroundIIFE

为什么？立即执行函数是独立单元，在它的括号中清楚的定义。注意在模块化的世界中，你几乎不需要立即执行函数。
```
// immediately-invoked function expression (IIFE)
(function () {
  console.log('Welcome to the Internet. Please follow me.');
}());
```

7.3 禁止在 (if, while等) 块中定义函数，将函数赋值给 var 变量。Assign the function to a variable instead. Browsers will allow you to do it, but they all interpret it differently, which is bad news bears. eslint: no-loop-func

7.4 注意: ECMA-262 定义代码块 block 是语包。函数声明不是语句。问题 ECMA-262's 关于这个问题的说明.

// bad
if (currentUser) {
  function test() {
    console.log('Nope.');
  }
}

// good
let test;
if (currentUser) {
  test = () => {
    console.log('Yup.');
  };
}

7.5 禁止命名参数名为 arguments。这将会取代内置对象 arguments。

// bad
function foo(name, options, arguments) {
  // ...
}

// good
function foo(name, options, args) {
  // ...
}

7.6 不要使用 arguments，使用可变参数 rest 参数（可变参数） ... 代替。 eslint: prefer-rest-params

为什么? ... 清楚的指明你要使用那个参数。rest 参数是真正的数组，而 arguments 是类数组。
```
// bad
function concatenateAll() {
  const args = Array.prototype.slice.call(arguments);
  return args.join('');
}

// good
function concatenateAll(...args) {
  return args.join('');
}
```

7.7 使用参数默认值语法，而不是改变函数参数。

// really bad
function handleThings(opts) {
  // No! We shouldn't mutate function arguments.
  // 不！我们不应该改变参数的值。
  // Double bad: if opts is falsy it'll be set to an object which may
  // 第二错：如果 opts 是一个可转为 false 的值（falsy），它会被设置为 object ，可能你就是要这么做，但这是会导致一些微秒的 bugs。
  // be what you want but it can introduce subtle bugs.
  opts = opts || {};
  // ...
}

// still bad
function handleThings(opts) {
  if (opts === void 0) {
    opts = {};
  }
  // ...
}

// good
function handleThings(opts = {}) {
  // ...
}

7.8 避免将可变表达式设为默认值。

为什么？这会让人困惑。

var b = 1;
// bad
function count(a = b++) {
  console.log(a);
}
count();  // 1
count();  // 2
count(3); // 3
count();  // 3

7.9 始终将有默认值的参数放在最后。

// bad
function handleThings(opts = {}, name) {
  // ...
}

// good
function handleThings(name, opts = {}) {
  // ...
}

7.10 禁止使用 Function 构造函数创建函数。 eslint: no-new-func

为什么？这样创建函数类似于使用 eval() 计算字符串，会导致漏洞。
```
// bad
var add = new Function('a', 'b', 'return a + b');

// still bad
var subtract = Function('a', 'b', 'return a - b');
```

7.11 在 function 关键字会添加空格。 eslint: space-before-function-paren space-before-blocks

为什么？统一格式，你不必在添加或是删除函数名时添加或删除空格。
```
// bad
const f = function(){};
const g = function (){};
const h = function() {};

// good
const x = function () {};
const y = function a() {};
```

7.12 禁止修改传入参数。 eslint: no-param-reassign

为什么？操作传入参数会改变调用者的原始值，引起不必要的副作用。

// bad
function f1(obj) {
  obj.key = 1;
}

// good
function f2(obj) {
  const key = Object.prototype.hasOwnProperty.call(obj, 'key') ? obj.key : 1;
}

7.13 禁止重新指定新值给传入参数。 eslint: no-param-reassign

为什么？重新指定新值给传入参数可能会导致意外的问题，特别是在访问 arguments 对象时。还会产生优化问题，特别是在 V8 引擎。
```
// bad
function f1(a) {
  a = 1;
  // ...
}

function f2(a) {
  if (!a) { a = 1; }
  // ...
}

// good
function f3(a) {
  const b = a || 1;
  // ...
}

function f4(a = 1) {
  // ...
}
```

7.14 使用展开运算符 ... 去调用接收可变值的函数。 eslint: prefer-spread

为什么？这样是清淅的，你不用指定上下文（this），不在需要使用 apply。

// bad
const x = [1, 2, 3, 4, 5];
console.log.apply(console, x);

// good
const x = [1, 2, 3, 4, 5];
console.log(...x);

// bad
new (Function.prototype.bind.apply(Date, [null, 2016, 8, 5]));

// good
new Date(...[2016, 8, 5]);

7.15 多行参数的函数需要缩进，像本指南中的其它多行列表一样，每行一个值，在最后一项上也需要有逗号。

// bad
function foo(bar,
             baz,
             quux) {
  // ...
}

// good
function foo(
  bar,
  baz,
  quux,
) {
  // ...
}

// bad
console.log(foo,
  bar,
  baz);

// good
console.log(
  foo,
  bar,
  baz,
);

⬆ 返回目录

箭头函数 Arrow Functions

8.1 当你必须使用函数表达式，（当传递匿名函数时），请使用箭头函数。eslint: prefer-arrow-callback, arrow-spacing jscs: requireArrowFunctions

为什么? 箭头函数的 this 是由定义位置的上下文确定的，这通常能达到你的需求，语法也会更简洁。

为什么不? 好果你有一个复杂的函数，你可以把逻辑定义到一个函数中。
```
// bad
[1, 2, 3].map(function (x) {
  const y = x + 1;
  return x * y;
});

// good
[1, 2, 3].map((x) => {
  const y = x + 1;
  return x * y;
});
```

8.2 如果函数内容只有一个表达式, 可以省略大括号，其它情况下应该保留大括号和 return 语句。 eslint: arrow-parens, arrow-body-style jscs: disallowParenthesesAroundArrowParam, requireShorthandArrowFunctions

为什么？这是语法糖。在链式调用中有很好的可读性。

// bad
[1, 2, 3].map(number => {
  const nextNumber = number + 1;
  `A string containing the ${nextNumber}.`;
});

// good
[1, 2, 3].map(number => `A string containing the ${number}.`);

// good
[1, 2, 3].map((number) => {
  const nextNumber = number + 1;
  return `A string containing the ${nextNumber}.`;
});

// good
[1, 2, 3].map((number, index) => ({
  [index]: number,
}));

8.3 函数体有多行时，用括号包裹以增加可读性。

为什么？这要可以清楚的展示函数的边界。

// bad
['get', 'post', 'put'].map(httpMethod => Object.prototype.hasOwnProperty.call(
    httpMagicObjectWithAVeryLongName,
    httpMethod,
  )
);

// good
['get', 'post', 'put'].map(httpMethod => (
  Object.prototype.hasOwnProperty.call(
    httpMagicObjectWithAVeryLongName,
    httpMethod,
  )
));

8.4 如果你的函数只有一个参数也不使用大括号，可以省略括号。其它情况使用括号包裹参数。 Note: it is also acceptable to always use parentheses, in which case use the "always" option for eslint or do not include disallowParenthesesAroundArrowParam for jscs. eslint: arrow-parens jscs: disallowParenthesesAroundArrowParam

为什么? 更清晰。

// bad
[1, 2, 3].map((x) => x * x);

// good
[1, 2, 3].map(x => x * x);

// good
[1, 2, 3].map(number => (
  `A long string with the ${number}. It’s so long that we don’t want it to take up space on the .map line!`
));

// bad
[1, 2, 3].map(x => {
  const y = x + 1;
  return x * y;
});

// good
[1, 2, 3].map((x) => {
  const y = x + 1;
  return x * y;
});

8.5 避免混淆箭头函数语法 (=>) 与比较运算符 (<=, >=)。 eslint: no-confusing-arrow

// bad
const itemHeight = item => item.height > 256 ? item.largeSize : item.smallSize;

// bad
const itemHeight = (item) => item.height > 256 ? item.largeSize : item.smallSize;

// good
const itemHeight = item => (item.height > 256 ? item.largeSize : item.smallSize);

// good
const itemHeight = (item) => {
  const { height, largeSize, smallSize } = item;
  return height > 256 ? largeSize : smallSize;
};

⬆ 返回目录

类 & 构造函数 Classes & Constructors

9.1 总是使用 class.避免直接使用 prototype。

为什么？class 语法更简洁更容易。

// bad
function Queue(contents = []) {
  this.queue = [...contents];
}
Queue.prototype.pop = function () {
  const value = this.queue[0];
  this.queue.splice(0, 1);
  return value;
};

// good
class Queue {
  constructor(contents = []) {
    this.queue = [...contents];
  }
  pop() {
    const value = this.queue[0];
    this.queue.splice(0, 1);
    return value;
  }
}

9.2 使用 extends 进行继承。

为什么? 它是内置的继承方法，不会破坏 instanceof。

// bad
const inherits = require('inherits');
function PeekableQueue(contents) {
  Queue.apply(this, contents);
}
inherits(PeekableQueue, Queue);
PeekableQueue.prototype.peek = function () {
  return this.queue[0];
};

// good
class PeekableQueue extends Queue {
  peek() {
    return this.queue[0];
  }
}

9.3 方法可以返回 this 来帮助进行链式调用。

// bad
Jedi.prototype.jump = function () {
  this.jumping = true;
  return true;
};

Jedi.prototype.setHeight = function (height) {
  this.height = height;
};

const luke = new Jedi();
luke.jump(); // => true
luke.setHeight(20); // => undefined

// good
class Jedi {
  jump() {
    this.jumping = true;
    return this;
  }

  setHeight(height) {
    this.height = height;
    return this;
  }
}

const luke = new Jedi();

luke.jump()
  .setHeight(20);

9.4 允许自定义 toString() 方法，只要能确保工作正常就可以。

class Jedi {
  constructor(options = {}) {
    this.name = options.name || 'no name';
  }

  getName() {
    return this.name;
  }

  toString() {
    return `Jedi - ${this.getName()}`;
  }
}

9.5 空构造函数或是只调用父类的空构造函数是没有必要的。 eslint: no-useless-constructor

// bad
class Jedi {
  constructor() {}

  getName() {
    return this.name;
  }
}

// bad
class Rey extends Jedi {
  constructor(...args) {
    super(...args);
  }
}

// good
class Rey extends Jedi {
  constructor(...args) {
    super(...args);
    this.name = 'Rey';
  }
}

9.6 避免重复类成员。 eslint: no-dupe-class-members

为什么？重复方法只会保留最后一个，有重复方法几乎必然会有 bug。

// bad
class Foo {
  bar() { return 1; }
  bar() { return 2; }
}

// good
class Foo {
  bar() { return 1; }
}

// good
class Foo {
  bar() { return 2; }
}

⬆ 返回目录

模块 Modules

10.1 始终使用模块化 (import/export) 。

为什么？模块化是新特性，开始使用它吧。

// bad
const AirbnbStyleGuide = require('./AirbnbStyleGuide');
module.exports = AirbnbStyleGuide.es6;

// ok
import AirbnbStyleGuide from './AirbnbStyleGuide';
export default AirbnbStyleGuide.es6;

// best
import { es6 } from './AirbnbStyleGuide';
export default es6;

10.2 不要使用通配符 import。

为什么？这确保你有一个默认的 export。

// bad
import * as AirbnbStyleGuide from './AirbnbStyleGuide';

// good
import AirbnbStyleGuide from './AirbnbStyleGuide';

10.3 不在一行复合写 export import.

为什么？一行虽然看起来简洁，但是一个清楚的 import 和一个清楚的 export 会使代码更加一致。

// bad
// filename es6.js
export { es6 as default } from './AirbnbStyleGuide';

// good
// filename es6.js
import { es6 } from './AirbnbStyleGuide';
export default es6;

10.4 同一路径只有一个地方 import。 eslint: no-duplicate-imports

为什么？多行 import 同一个路径会使代码难以维护。

// bad
import foo from 'foo';
// … some other imports … //
import { named1, named2 } from 'foo';

// good
import foo, { named1, named2 } from 'foo';

// good
import foo, {
  named1,
  named2,
} from 'foo';

10.5 不 export 可变的数据。 eslint: import/no-mutable-exports

Why? Mutation should be avoided in general, but in particular when exporting mutable bindings. While this technique may be needed for some special cases, in general, only constant references should be exported.
```
// bad
let foo = 3;
export { foo };

// good
const foo = 3;
export { foo };
```

10.6 模块只有一个 export 时，使用 default。 eslint: import/prefer-default-export
```
// bad
export function foo() {}

// good
export default function foo() {}
```

10.7 把所有 import 放到顶部. eslint: import/first

为什么？ import 有变量提升作用，将它们放到顶部可以防止出来意外行为。

// bad
import foo from 'foo';
foo.init();

import bar from 'bar';

// good
import foo from 'foo';
import bar from 'bar';

foo.init();

10.8 多行 import 应该像对象或者数组定义一样进行缩进。

为什么？遵循相同的缩进几格。

// bad
import {longNameA, longNameB, longNameC, longNameD, longNameE} from 'path';

// good
import {
  longNameA,
  longNameB,
  longNameC,
  longNameD,
  longNameE,
} from 'path';

10.9 不允许在 import 中使用 webpack loader 语法. eslint: import/no-webpack-loader-syntax

为什么？在 import 中什么 loader 会增加耦合度，应该放到 webpack.config.js 中去。
```
// bad
import fooSass from 'css!sass!foo.scss';
import barCss from 'style!css!bar.css';

// good
import fooSass from 'foo.scss';
import barCss from 'bar.css';
```

⬆ 返回目录

迭代器和Generators Iterators and Generators

11.1 不要使用迭代器，而是使用 JavaScript 的高价函数,以替代 for-in or for-of。 eslint: no-iterator no-restricted-syntax

为什么？这是我们不变的规则。Dealing with pure functions that return values is easier to reason about than side effects.

Use map() / every() / filter() / find() / findIndex() / reduce() / some() / ... to iterate over arrays, and Object.keys() / Object.values() / Object.entries() to produce arrays so you can iterate over objects.

const numbers = [1, 2, 3, 4, 5];

// bad
let sum = 0;
for (let num of numbers) {
  sum += num;
}
sum === 15;

// good
let sum = 0;
numbers.forEach(num => sum += num);
sum === 15;

// best (use the functional force)
const sum = numbers.reduce((total, num) => total + num, 0);
sum === 15;

// bad
const increasedByOne = [];
for (let i = 0; i < numbers.length; i++) {
  increasedByOne.push(numbers[i] + 1);
}

// good
const increasedByOne = [];
numbers.forEach(num => increasedByOne.push(num + 1));

// best (keeping it functional)
const increasedByOne = numbers.map(num => num + 1);

11.2 现在这个时刻不要使用 generators.

为什么？现在不能很好的转换为 ES5

11.3 如果你必须使用 generators ，或者忽视上一条规则 our advice, 确保使用正确的分隔。 eslint: generator-star-spacing

为什么? function 和 *是同一个语法的一部分 * 不是 function 的修饰符, function* 是一个独特的结构, 跟 function 不同。

// bad
function * foo() {
  // ...
}

// bad
const bar = function * () {
  // ...
};

// bad
const baz = function *() {
  // ...
};

// bad
const quux = function*() {
  // ...
};

// bad
function*foo() {
  // ...
}

// bad
function *foo() {
  // ...
}

// very bad
function
*
foo() {
  // ...
}

// very bad
const wat = function
*
() {
  // ...
};

// good
function* foo() {
  // ...
}

// good
const foo = function* () {
  // ...
};

⬆ 返回目录

Properties

12.1 使用 . 来访问属性。 eslint: dot-notation jscs: requireDotNotation

const luke = {
  jedi: true,
  age: 28,
};

// bad
const isJedi = luke['jedi'];

// good
const isJedi = luke.jedi;

12.2 当属性名是变量时，使用 [] 来访问。

const luke = {
  jedi: true,
  age: 28,
};

function getProp(prop) {
  return luke[prop];
}

const isJedi = getProp('jedi');

⬆ 返回目录

变量 Variables

13.1 总是使用 const 或者 let 声明变量. 不这么做会得到一个全局变量。我们要避免污染全局命名空间。 Captain Planet warned us of that. eslint: no-undef prefer-const
```
// bad
superPower = new SuperPower();

// good
const superPower = new SuperPower();
```

13.2 每次使用 const 或者 let 声明一个变量。 eslint: one-var jscs: disallowMultipleVarDecl

为什么? 这样容易添加新的变量声明，你永远不用担心把 ; 更换为 ,，或者引入只有标点的 diffs。在调试时可以遍历每个变量声明，而不是一次跳过他们。

// bad
const items = getItems(),
    goSportsTeam = true,
    dragonball = 'z';

// bad
// (compare to above, and try to spot the mistake)
const items = getItems(),
    goSportsTeam = true;
    dragonball = 'z';

// good
const items = getItems();
const goSportsTeam = true;
const dragonball = 'z';

13.3 把所有的 const 和 let 分级放到一起。

为什么?这有助于之后声明变量依赖之前的声明。

// bad
let i, len, dragonball,
    items = getItems(),
    goSportsTeam = true;

// bad
let i;
const items = getItems();
let dragonball;
const goSportsTeam = true;
let len;

// good
const goSportsTeam = true;
const items = getItems();
let dragonball;
let i;
let length;

13.4 当你需要的时候分配变量，但是把他们放在合理的地方。

为什么? let 和 const 是块级作用域，不是函数作用域。

// bad - unnecessary function call
function checkName(hasName) {
  const name = getName();

  if (hasName === 'test') {
    return false;
  }

  if (name === 'test') {
    this.setName('');
    return false;
  }

  return name;
}

// good
function checkName(hasName) {
  if (hasName === 'test') {
    return false;`
  }

  const name = getName();

  if (name === 'test') {
    this.setName('');
    return false;
  }

  return name;
}

13.5 禁止链式声明变量.

为什么?链式声明会创建隐式的全局变量。

// bad
(function example() {
  // JavaScript interprets this as
  // let a = ( b = ( c = 1 ) );
  // The let keyword only applies to variable a; variables b and c become
  // global variables.
  let a = b = c = 1;
}());

console.log(a); // undefined
console.log(b); // 1
console.log(c); // 1

// good
(function example() {
  let a = 1;
  let b = a;
  let c = a;
}());

console.log(a); // undefined
console.log(b); // undefined
console.log(c); // undefined

// the same applies for `const`

13.6 避免使用 ++, --。 eslint no-plusplus

为什么? 在 eslint 文档中，一元操作符 ++ 和 -- 会受到自动插入分号的影响，还会导致无声的错误。使用 num += 1 代替 num++ 或者 num ++ 会更有表现力。禁止使用 ++ -- 还会避免你无意的增加减少值。

// bad

const array = [1, 2, 3];
let num = 1;
num++;
--num;

let sum = 0;
let truthyCount = 0;
for (let i = 0; i < array.length; i++) {
  let value = array[i];
  sum += value;
  if (value) {
    truthyCount++;
  }
}

// good

const array = [1, 2, 3];
let num = 1;
num += 1;
num -= 1;

const sum = array.reduce((a, b) => a + b, 0);
const truthyCount = array.filter(Boolean).length;

⬆ 返回目录

提升 Hoisting

14.1 var declarations get hoisted to the top of their scope, their assignment does not. const and let declarations are blessed with a new concept called Temporal Dead Zones (TDZ). It's important to know why typeof is no longer safe.

14.1 var 会把变量声明提升到作用域的顶部，初始化不会。const 和 let 声明有一个新名词叫作暂时性死区 Temporal Dead Zones (TDZ). 这有助于理解为什么 typeof 不在是安全的。(http://es-discourse.com/t/why-typeof-is-no-longer-safe/15).

// 我们知道这是不会正常工作的 (假设没有在全局作用域进行声明)
function example() {
  console.log(notDefined); // => throws a ReferenceError
}

// 声明一个变量在使用他之后，变量的声明会提升
// 他的值 `true` 不会提升
function example() {
  console.log(declaredButNotAssigned); // => undefined
  var declaredButNotAssigned = true;
}

// 上面的例子和下面的一样
function example() {
  let declaredButNotAssigned;
  console.log(declaredButNotAssigned); // => undefined
  declaredButNotAssigned = true;
}

// using const and let
function example() {
  console.log(declaredButNotAssigned); // => throws a ReferenceError
  console.log(typeof declaredButNotAssigned); // => throws a ReferenceError
  const declaredButNotAssigned = true;
}

14.2 匿名函数会会提升他的直接量，但是不提升函数定义。

function example() {
  console.log(anonymous); // => undefined

  anonymous(); // => TypeError anonymous is not a function

  var anonymous = function () {
    console.log('anonymous function expression');
  };
}

14.3 命名函数赋值给变量提升命名, 定义不会提升。

function example() {
  console.log(named); // => undefined

  named(); // => TypeError named is not a function

  superPower(); // => ReferenceError superPower is not defined

  var named = function superPower() {
    console.log('Flying');
  };
}

// the same is true when the function name
// is the same as the variable name.
function example() {
  console.log(named); // => undefined

  named(); // => TypeError named is not a function

  var named = function named() {
    console.log('named');
  };
}

14.4 Function 语句定义会提升变量和定义。

function example() {
  superPower(); // => Flying

  function superPower() {
    console.log('Flying');
  }
}

查看更多 JavaScript 作用域和提升 JavaScript Scoping & Hoisting by Ben Cherry.

⬆ 返回目录

比较运算符 & 等号 Comparison Operators & Equality

15.1 使用 === 和 !== 代替 == 和 !=. eslint: eqeqeq

15.2 判断语句比如 if 强制使用 ToBoolean 来对比它们，并且并是遵守下面的规则：
- Objects 转换为 true
- Undefined 转换为 false
- Null e转换为 false
- Booleans 转换为 他的 boolean 值
- Numbers if +0, -0, or NaN 转换为 false, 其它情况转换为 true
- Strings if an empty string '' 转换为 false , 其它情况转换为 true
```
if ([0] && []) {
  // true
  // an array (even an empty one) is an object, objects will evaluate to true
}
```

15.3 对 booleans 使用简短语句，对字符串和数字使用显示比较。

// bad
if (isValid === true) {
  // ...
}

// good
if (isValid) {
  // ...
}

// bad
if (name) {
  // ...
}

// good
if (name !== '') {
  // ...
}

// bad
if (collection.length) {
  // ...
}

// good
if (collection.length > 0) {
  // ...
}

15.4 更多信息查看 Truth Equality and JavaScript by Angus Croll.

15.5 Use braces to create blocks in case and default clauses that contain lexical declarations (e.g. let, const, function, and class).

Why? Lexical declarations are visible in the entire switch block but only get initialized when assigned, which only happens when its case is reached. This causes problems when multiple case clauses attempt to define the same thing.

eslint rules: no-case-declarations.

// bad
switch (foo) {
  case 1:
    let x = 1;
    break;
  case 2:
    const y = 2;
    break;
  case 3:
    function f() {
      // ...
    }
    break;
  default:
    class C {}
}

// good
switch (foo) {
  case 1: {
    let x = 1;
    break;
  }
  case 2: {
    const y = 2;
    break;
  }
  case 3: {
    function f() {
      // ...
    }
    break;
  }
  case 4:
    bar();
    break;
  default: {
    class C {}
  }
}

15.6 Ternaries should not be nested and generally be single line expressions.

eslint rules: no-nested-ternary.

// bad
const foo = maybe1 > maybe2
  ? "bar"
  : value1 > value2 ? "baz" : null;

// better
const maybeNull = value1 > value2 ? 'baz' : null;

const foo = maybe1 > maybe2
  ? 'bar'
  : maybeNull;

// best
const maybeNull = value1 > value2 ? 'baz' : null;

const foo = maybe1 > maybe2 ? 'bar' : maybeNull;

15.7 避免不必要的三元运算。

eslint rules: no-unneeded-ternary.

// bad
const foo = a ? a : b;
const bar = c ? true : false;
const baz = c ? false : true;

// good
const foo = a || b;
const bar = !!c;
const baz = !c;

⬆ 返回目录

块 Blocks

16.1 对多行代码使用大括号。

// bad
if (test)
  return false;

// good
if (test) return false;

// good
if (test) {
  return false;
}

// bad
function foo() { return false; }

// good
function bar() {
  return false;
}

16.2 如果 if 或者 else 语句中有多行，把 else 放在 if 结束大括号的后面。 eslint: brace-style jscs: disallowNewlineBeforeBlockStatements

// bad
if (test) {
  thing1();
  thing2();
}
else {
  thing3();
}

// good
if (test) {
  thing1();
  thing2();
} else {
  thing3();
}

⬆ 返回目录

Control Statements

17.1 In case your control statement (if, while etc.) gets too long or exceeds the maximum line length, each (grouped) condition could be put into a new line. It's up to you whether the logical operator should begin or end the line.

// bad
if ((foo === 123 || bar === 'abc') && doesItLookGoodWhenItBecomesThatLong() && isThisReallyHappening()) {
  thing1();
}

// bad
if (foo === 123 &&
  bar === 'abc') {
  thing1();
}

// bad
if (foo === 123
  && bar === 'abc') {
  thing1();
}

// good
if (
  (foo === 123 || bar === "abc") &&
  doesItLookGoodWhenItBecomesThatLong() &&
  isThisReallyHappening()
) {
  thing1();
}

// good
if (foo === 123 && bar === 'abc') {
  thing1();
}

// good
if (
  foo === 123 &&
  bar === 'abc'
) {
  thing1();
}

// good
if (
  foo === 123
  && bar === 'abc'
) {
  thing1();
}

⬆ back to top

注释 Comments

17.1 多行注释使用 /** ... */。

// bad
// make() returns a new element
// based on the passed in tag name
//
// @param {String} tag
// @return {Element} element
function make(tag) {

  // ...

  return element;
}

// good
/**
 * make() returns a new element
 * based on the passed-in tag name
 */
function make(tag) {

  // ...

  return element;
}

18.2 单行注释，使用 //。单行注释放在所注释内容上方单独的新行中，注释上面空一行，除了注释是块中的第一行的情况下。

// bad
const active = true;  // is current tab

// good
// is current tab
const active = true;

// bad
function getType() {
  console.log('fetching type...');
  // set the default type to 'no type'
  const type = this.type || 'no type';

  return type;
}

// good
function getType() {
  console.log('fetching type...');

  // set the default type to 'no type'
  const type = this.type || 'no type';

  return type;
}

// also good
function getType() {
  // set the default type to 'no type'
  const type = this.type || 'no type';

  return type;
}

18.3 在注释开头使用空格以使注释易于阅读。 eslint: spaced-comment

// bad
//is current tab
const active = true;

// good
// is current tab
const active = true;

// bad
/**
 *make() returns a new element
 *based on the passed-in tag name
 */
function make(tag) {

  // ...

  return element;
}

// good
/**
 * make() returns a new element
 * based on the passed-in tag name
 */
function make(tag) {

  // ...

  return element;
}

18.4 使用 FIXME 或者 TODO 来让其它开发者理解代码有问题需要被解决，或者你有解决方案需要被实现。这与普通注释不同，因为它们是能付诸实施，FIXME: -- 需要修复 or TODO: -- 需要实现.

18.5 Use // FIXME: to annotate problems.

class Calculator extends Abacus {
  constructor() {
    super();

    // FIXME: shouldn't use a global here
    total = 0;
  }
}

18.6 Use // TODO: to annotate solutions to problems.

class Calculator extends Abacus {
  constructor() {
    super();

    // TODO: total should be configurable by an options param
    this.total = 0;
  }
}

⬆ back to top

空白 Whitespace

19.1 把 tabs 设置为两个空格。 eslint: indent jscs: validateIndentation

// bad
function foo() {
∙∙∙∙let name;
}

// bad
function bar() {
∙let name;
}

// good
function baz() {
∙∙let name;
}

19.2 在大括号前填加一个空格。 eslint: space-before-blocks jscs: requireSpaceBeforeBlockStatements

// bad
function test(){
  console.log('test');
}

// good
function test() {
  console.log('test');
}

// bad
dog.set('attr',{
  age: '1 year',
  breed: 'Bernese Mountain Dog',
});

// good
dog.set('attr', {
  age: '1 year',
  breed: 'Bernese Mountain Dog',
});

19.3 在 (if, while 等) 的圆括号前填加一个空格，方法名和方法参数圆括号之间不要填加空格。 eslint: keyword-spacing jscs: requireSpaceAfterKeywords

// bad
if(isJedi) {
  fight ();
}

// good
if (isJedi) {
  fight();
}

// bad
function fight () {
  console.log ('Swooosh!');
}

// good
function fight() {
  console.log('Swooosh!');
}

19.4 在操作符和操作数之间填加空格。 eslint: space-infix-ops jscs: requireSpaceBeforeBinaryOperators, requireSpaceAfterBinaryOperators
```
// bad
const x=y+5;

// good
const x = y + 5;
```

19.5 文件结尾使用单个换行。 eslint: eol-last

// bad
import { es6 } from './AirbnbStyleGuide';
  // ...
export default es6;

// bad
import { es6 } from './AirbnbStyleGuide';
  // ...
export default es6;↵
↵

// good
import { es6 } from './AirbnbStyleGuide';
  // ...
export default es6;↵

19.6 对超过 2 个调用的长方法链使用缩进。把点写在前面，突出是方法调用面不是新语句。 eslint: newline-per-chained-call no-whitespace-before-property

// bad
$('#items').find('.selected').highlight().end().find('.open').updateCount();

// bad
$('#items').
  find('.selected').
    highlight().
    end().
  find('.open').
    updateCount();

// good
$('#items')
  .find('.selected')
    .highlight()
    .end()
  .find('.open')
    .updateCount();

// bad
const leds = stage.selectAll('.led').data(data).enter().append('svg:svg').classed('led', true)
    .attr('width', (radius + margin) * 2).append('svg:g')
    .attr('transform', `translate(${radius + margin},${radius + margin})`)
    .call(tron.led);

// good
const leds = stage.selectAll('.led')
    .data(data)
  .enter().append('svg:svg')
    .classed('led', true)
    .attr('width', (radius + margin) * 2)
  .append('svg:g')
    .attr('transform', `translate(${radius + margin},${radius + margin})`)
    .call(tron.led);

// good
const leds = stage.selectAll('.led').data(data);

19.7 在块和下一个语句中留出一个空行。 jscs: requirePaddingNewLinesAfterBlocks

// bad
if (foo) {
  return bar;
}
return baz;

// good
if (foo) {
  return bar;
}

return baz;

// bad
const obj = {
  foo() {
  },
  bar() {
  },
};
return obj;

// good
const obj = {
  foo() {
  },

  bar() {
  },
};

return obj;

// bad
const arr = [
  function foo() {
  },
  function bar() {
  },
];
return arr;

// good
const arr = [
  function foo() {
  },

  function bar() {
  },
];

return arr;

19.8 不要在块中填充空白。 eslint: padded-blocks jscs: disallowPaddingNewlinesInBlocks

// bad
function bar() {

  console.log(foo);

}

// also bad
if (baz) {

  console.log(qux);
} else {
  console.log(foo);

}

// good
function bar() {
  console.log(foo);
}

// good
if (baz) {
  console.log(qux);
} else {
  console.log(foo);
}

19.9 不要在括号中者填加空格。 eslint: space-in-parens jscs: disallowSpacesInsideParentheses

// bad
function bar( foo ) {
  return foo;
}

// good
function bar(foo) {
  return foo;
}

// bad
if ( foo ) {
  console.log(foo);
}

// good
if (foo) {
  console.log(foo);
}

19.10 不要在中括号中填加空格。 eslint: array-bracket-spacing jscs: disallowSpacesInsideArrayBrackets

// bad
const foo = [ 1, 2, 3 ];
console.log(foo[ 0 ]);

// good
const foo = [1, 2, 3];
console.log(foo[0]);

19.11 在大括号中要填加空格。 eslint: object-curly-spacing jscs: requireSpacesInsideObjectBrackets
```
// bad
const foo = {clark: 'kent'};

// good
const foo = { clark: 'kent' };
```

19.12 避免在一行代码中超过 100 个字符 (包括空格). Note: per above, 长字符串不在限制范围内。 eslint: max-len jscs: maximumLineLength

为什么？这会保证代码可读性和可维护性。

// bad
const foo = jsonData && jsonData.foo && jsonData.foo.bar && jsonData.foo.bar.baz && jsonData.foo.bar.baz.quux && jsonData.foo.bar.baz.quux.xyzzy;

// bad
$.ajax({ method: 'POST', url: 'https://airbnb.com/', data: { name: 'John' } }).done(() => console.log('Congratulations!')).fail(() => console.log('You have failed this city.'));

// good
const foo = jsonData
  && jsonData.foo
  && jsonData.foo.bar
  && jsonData.foo.bar.baz
  && jsonData.foo.bar.baz.quux
  && jsonData.foo.bar.baz.quux.xyzzy;

// good
$.ajax({
  method: 'POST',
  url: 'https://airbnb.com/',
  data: { name: 'John' },
})
  .done(() => console.log('Congratulations!'))
  .fail(() => console.log('You have failed this city.'));

⬆ 返回目录

逗号 Commas

20.1 开头的逗号: Nope. eslint: comma-style jscs: requireCommaBeforeLineBreak

// bad
const story = [
    once
  , upon
  , aTime
];

// good
const story = [
  once,
  upon,
  aTime,
];

// bad
const hero = {
    firstName: 'Ada'
  , lastName: 'Lovelace'
  , birthYear: 1815
  , superPower: 'computers'
};

// good
const hero = {
  firstName: 'Ada',
  lastName: 'Lovelace',
  birthYear: 1815,
  superPower: 'computers',
};

20.2 额外的逗号: Yup. eslint: comma-dangle jscs: requireTrailingComma

为什么？为了方便 git diffs。同时，转换工具像 Babel 等会删除额外的逗号，这意味着你不用担心在低版本的浏览器中额外的逗号导致问题。trailing comma problem

// bad - git diff without trailing comma
const hero = {
     firstName: 'Florence',
-    lastName: 'Nightingale'
+    lastName: 'Nightingale',
+    inventorOf: ['coxcomb chart', 'modern nursing']
};

// good - git diff with trailing comma
const hero = {
     firstName: 'Florence',
     lastName: 'Nightingale',
+    inventorOf: ['coxcomb chart', 'modern nursing'],
};

// bad
const hero = {
  firstName: 'Dana',
  lastName: 'Scully'
};

const heroes = [
  'Batman',
  'Superman'
];

// good
const hero = {
  firstName: 'Dana',
  lastName: 'Scully',
};

const heroes = [
  'Batman',
  'Superman',
];

// bad
function createHero(
  firstName,
  lastName,
  inventorOf
) {
  // does nothing
}

// good
function createHero(
  firstName,
  lastName,
  inventorOf,
) {
  // does nothing
}

// good (note that a comma must not appear after a "rest" element)
function createHero(
  firstName,
  lastName,
  inventorOf,
  ...heroArgs
) {
  // does nothing
}

// bad
createHero(
  firstName,
  lastName,
  inventorOf
);

// good
createHero(
  firstName,
  lastName,
  inventorOf,
);

// good (note that a comma must not appear after a "rest" element)
createHero(
  firstName,
  lastName,
  inventorOf,
  ...heroArgs
);

⬆ 返回目录

分号 Semicolons

21.1 Yup. eslint: semi jscs: requireSemicolons

// bad
(function () {
  const name = 'Skywalker'
  return name
})()

// good
(function () {
  const name = 'Skywalker';
  return name;
}());

// good, but legacy (guards against the function becoming an argument when two files with IIFEs are concatenated)
;((() => {
  const name = 'Skywalker';
  return name;
})());

类型转换 Type Casting & Coercion

21.1 在语句开始的时候执行类型转换。

22.2 Strings:

// => this.reviewScore = 9;

// bad
const totalScore = this.reviewScore + ''; // invokes this.reviewScore.valueOf()

// bad
const totalScore = this.reviewScore.toString(); // isn't guaranteed to return a string

// good
const totalScore = String(this.reviewScore);

22.3 Numbers: 使用 Number 和带基数的 parseInt 进行 String 转 Number 类型。eslint: radix

const inputValue = '4';

// bad
const val = new Number(inputValue);

// bad
const val = +inputValue;

// bad
const val = inputValue >> 0;

// bad
const val = parseInt(inputValue);

// good
const val = Number(inputValue);

// good
const val = parseInt(inputValue, 10);

22.4 如果你在做一些事情，parseInt 成为了瓶颈，你需要什么位移来改善性能，请写清楚相关注释 performance reasons。 performance reasons, leave a comment explaining why and what you're doing.
```
// good
/**
 * parseInt was the reason my code was slow.
 * Bitshifting the String to coerce it to a
 * Number made it a lot faster.
 */
const val = inputValue >> 0;
```

22.5 Note: Be careful when using bitshift operations. Numbers are represented as 64-bit values, but bitshift operations always return a 32-bit integer (source). Bitshift can lead to unexpected behavior for integer values larger than 32 bits. Discussion. Largest signed 32-bit Int is 2,147,483,647:
```
2147483647 >> 0; // => 2147483647
2147483648 >> 0; // => -2147483648
2147483649 >> 0; // => -2147483647
```

22.6 Booleans:

const age = 0;

// bad
const hasAge = new Boolean(age);

// good
const hasAge = Boolean(age);

// best
const hasAge = !!age;

⬆ 返回目录

Naming Conventions

23.1 Avoid single letter names. Be descriptive with your naming. eslint: id-length

// bad
function q() {
  // ...
}

// good
function query() {
  // ...
}

23.2 Use camelCase when naming objects, functions, and instances. eslint: camelcase jscs: requireCamelCaseOrUpperCaseIdentifiers

// bad
const OBJEcttsssss = {};
const this_is_my_object = {};
function c() {}

// good
const thisIsMyObject = {};
function thisIsMyFunction() {}

23.3 Use PascalCase only when naming constructors or classes. eslint: new-cap jscs: requireCapitalizedConstructors

// bad
function user(options) {
  this.name = options.name;
}

const bad = new user({
  name: 'nope',
});

// good
class User {
  constructor(options) {
    this.name = options.name;
  }
}

const good = new User({
  name: 'yup',
});

23.4 Do not use trailing or leading underscores. eslint: no-underscore-dangle jscs: disallowDanglingUnderscores

Why? JavaScript does not have the concept of privacy in terms of properties or methods. Although a leading underscore is a common convention to mean “private”, in fact, these properties are fully public, and as such, are part of your public API contract. This convention might lead developers to wrongly think that a change won't count as breaking, or that tests aren't needed. tl;dr: if you want something to be “private”, it must not be observably present.
```
// bad
this.__firstName__ = 'Panda';
this.firstName_ = 'Panda';
this._firstName = 'Panda';

// good
this.firstName = 'Panda';
```

23.5 Don't save references to this. Use arrow functions or Function#bind. jscs: disallowNodeTypes

// bad
function foo() {
  const self = this;
  return function () {
    console.log(self);
  };
}

// bad
function foo() {
  const that = this;
  return function () {
    console.log(that);
  };
}

// good
function foo() {
  return () => {
    console.log(this);
  };
}

23.6 A base filename should exactly match the name of its default export.

// file 1 contents
class CheckBox {
  // ...
}
export default CheckBox;

// file 2 contents
export default function fortyTwo() { return 42; }

// file 3 contents
export default function insideDirectory() {}

// in some other file
// bad
import CheckBox from './checkBox'; // PascalCase import/export, camelCase filename
import FortyTwo from './FortyTwo'; // PascalCase import/filename, camelCase export
import InsideDirectory from './InsideDirectory'; // PascalCase import/filename, camelCase export

// bad
import CheckBox from './check_box'; // PascalCase import/export, snake_case filename
import forty_two from './forty_two'; // snake_case import/filename, camelCase export
import inside_directory from './inside_directory'; // snake_case import, camelCase export
import index from './inside_directory/index'; // requiring the index file explicitly
import insideDirectory from './insideDirectory/index'; // requiring the index file explicitly

// good
import CheckBox from './CheckBox'; // PascalCase export/import/filename
import fortyTwo from './fortyTwo'; // camelCase export/import/filename
import insideDirectory from './insideDirectory'; // camelCase export/import/directory name/implicit "index"
// ^ supports both insideDirectory.js and insideDirectory/index.js

23.7 Use camelCase when you export-default a function. Your filename should be identical to your function's name.
```
function makeStyleGuide() {
  // ...
}

export default makeStyleGuide;
```

23.8 Use PascalCase when you export a constructor / class / singleton / function library / bare object.
```
const AirbnbStyleGuide = {
  es6: {
  },
};

export default AirbnbStyleGuide;
```

23.9 Acronyms and initialisms should always be all capitalized, or all lowercased.

Why? Names are for readability, not to appease a computer algorithm.

// bad
import SmsContainer from './containers/SmsContainer';

// bad
const HttpRequests = [
  // ...
];

// good
import SMSContainer from './containers/SMSContainer';

// good
const HTTPRequests = [
  // ...
];

// best
import TextMessageContainer from './containers/TextMessageContainer';

// best
const Requests = [
  // ...
];

⬆ back to top

Accessors

24.1 Accessor functions for properties are not required.

24.2 Do not use JavaScript getters/setters as they cause unexpected side effects and are harder to test, maintain, and reason about. Instead, if you do make accessor functions, use getVal() and setVal('hello').

// bad
class Dragon {
  get age() {
    // ...
  }

  set age(value) {
    // ...
  }
}

// good
class Dragon {
  getAge() {
    // ...
  }

  setAge(value) {
    // ...
  }
}

24.3 If the property/method is a boolean, use isVal() or hasVal().

// bad
if (!dragon.age()) {
  return false;
}

// good
if (!dragon.hasAge()) {
  return false;
}

24.4 It's okay to create get() and set() functions, but be consistent.

class Jedi {
  constructor(options = {}) {
    const lightsaber = options.lightsaber || 'blue';
    this.set('lightsaber', lightsaber);
  }

  set(key, val) {
    this[key] = val;
  }

  get(key) {
    return this[key];
  }
}

⬆ back to top

Events

25.1 When attaching data payloads to events (whether DOM events or something more proprietary like Backbone events), pass a hash instead of a raw value. This allows a subsequent contributor to add more data to the event payload without finding and updating every handler for the event. For example, instead of:

// bad
$(this).trigger('listingUpdated', listing.id);

// ...

$(this).on('listingUpdated', (e, listingId) => {
  // do something with listingId
});

prefer:

// good
$(this).trigger('listingUpdated', { listingId: listing.id });

// ...

$(this).on('listingUpdated', (e, data) => {
  // do something with data.listingId
});

⬆ back to top

jQuery

26.1 Prefix jQuery object variables with a $. jscs: requireDollarBeforejQueryAssignment

// bad
const sidebar = $('.sidebar');

// good
const $sidebar = $('.sidebar');

// good
const $sidebarBtn = $('.sidebar-btn');

26.2 Cache jQuery lookups.

// bad
function setSidebar() {
  $('.sidebar').hide();

  // ...

  $('.sidebar').css({
    'background-color': 'pink',
  });
}

// good
function setSidebar() {
  const $sidebar = $('.sidebar');
  $sidebar.hide();

  // ...

  $sidebar.css({
    'background-color': 'pink',
  });
}

26.3 For DOM queries use Cascading $('.sidebar ul') or parent > child $('.sidebar > ul'). jsPerf

26.4 Use find with scoped jQuery object queries.

// bad
$('ul', '.sidebar').hide();

// bad
$('.sidebar').find('ul').hide();

// good
$('.sidebar ul').hide();

// good
$('.sidebar > ul').hide();

// good
$sidebar.find('ul').hide();

⬆ back to top

ECMAScript 5 Compatibility

27.1 Refer to Kangax's ES5 compatibility table.

⬆ back to top

ECMAScript 6+ (ES 2015+) Styles

28.1 This is a collection of links to the various ES6 features.

Arrow Functions
Classes
Object Shorthand
Object Concise
Object Computed Properties
Template Strings
Destructuring
Default Parameters
Rest
Array Spreads
Let and Const
Iterators and Generators
Modules

28.2 Do not use TC39 proposals that have not reached stage 3.

Why? They are not finalized, and they are subject to change or to be withdrawn entirely. We want to use JavaScript, and proposals are not JavaScript yet.

⬆ back to top

Testing

29.1 Yup.
```
function foo() {
  return true;
}
```

29.2 No, but seriously:
- Whichever testing framework you use, you should be writing tests!
- Strive to write many small pure functions, and minimize where mutations occur.
- Be cautious about stubs and mocks - they can make your tests more brittle.
- We primarily use mocha at Airbnb. tape is also used occasionally for small, separate modules.
- 100% test coverage is a good goal to strive for, even if it's not always practical to reach it.
- Whenever you fix a bug, write a regression test. A bug fixed without a regression test is almost certainly going to break again in the future.

⬆ back to top

Performance

⬆ back to top

Resources

Learning ES6

Read This

Standard ECMA-262

Tools

Code Style Linters
- ESlint - Airbnb Style .eslintrc
- JSHint - Airbnb Style .jshintrc
- JSCS - Airbnb Style Preset (Deprecated, please use ESlint)
Neutrino preset - neutrino-preset-airbnb-base

Other Style Guides

Other Styles

Naming this in nested functions - Christian Johansen
Conditional Callbacks - Ross Allen
Popular JavaScript Coding Conventions on GitHub - JeongHoon Byun
Multiple var statements in JavaScript, not superfluous - Ben Alman

Further Reading

Understanding JavaScript Closures - Angus Croll
Basic JavaScript for the impatient programmer - Dr. Axel Rauschmayer
You Might Not Need jQuery - Zack Bloom & Adam Schwartz
ES6 Features - Luke Hoban
Frontend Guidelines - Benjamin De Cock

Books

JavaScript: The Good Parts - Douglas Crockford
JavaScript Patterns - Stoyan Stefanov
Pro JavaScript Design Patterns - Ross Harmes and Dustin Diaz
High Performance Web Sites: Essential Knowledge for Front-End Engineers - Steve Souders
Maintainable JavaScript - Nicholas C. Zakas
JavaScript Web Applications - Alex MacCaw
Pro JavaScript Techniques - John Resig
Smashing Node.js: JavaScript Everywhere - Guillermo Rauch
Secrets of the JavaScript Ninja - John Resig and Bear Bibeault
Human JavaScript - Henrik Joreteg
Superhero.js - Kim Joar Bekkelund, Mads Mobæk, & Olav Bjorkoy
JSBooks - Julien Bouquillon
Third Party JavaScript - Ben Vinegar and Anton Kovalyov
Effective JavaScript: 68 Specific Ways to Harness the Power of JavaScript - David Herman
Eloquent JavaScript - Marijn Haverbeke
You Don't Know JS: ES6 & Beyond - Kyle Simpson

Blogs

Podcasts

⬆ back to top

In the Wild

This is a list of organizations that are using this style guide. Send us a pull request and we'll add you to the list.

3blades: 3Blades/javascript
4Catalyzer: 4Catalyzer/javascript
Aan Zee: AanZee/javascript
Adult Swim: adult-swim/javascript
Airbnb: airbnb/javascript
AltSchool: AltSchool/javascript
Apartmint: apartmint/javascript
Ascribe: ascribe/javascript
Avalara: avalara/javascript
Avant: avantcredit/javascript
Axept: axept/javascript
BashPros: BashPros/javascript
Billabong: billabong/javascript
Bisk: bisk/javascript
Bonhomme: bonhommeparis/javascript
Brainshark: brainshark/javascript
CaseNine: CaseNine/javascript
Chartboost: ChartBoost/javascript-style-guide
ComparaOnline: comparaonline/javascript
Compass Learning: compasslearning/javascript-style-guide
DailyMotion: dailymotion/javascript
DoSomething: DoSomething/eslint-config
Digitpaint digitpaint/javascript
Ecosia: ecosia/javascript
Evernote: evernote/javascript-style-guide
Evolution Gaming: evolution-gaming/javascript
EvozonJs: evozonjs/javascript
ExactTarget: ExactTarget/javascript
Expensify Expensify/Style-Guide
Flexberry: Flexberry/javascript-style-guide
Gawker Media: gawkermedia/javascript
General Electric: GeneralElectric/javascript
Generation Tux: GenerationTux/javascript
GoodData: gooddata/gdc-js-style
Grooveshark: grooveshark/javascript
Honey: honeyscience/javascript
How About We: howaboutwe/javascript
Huballin: huballin/javascript
HubSpot: HubSpot/javascript
Hyper: hyperoslo/javascript-playbook
InterCity Group: intercitygroup/javascript-style-guide
Jam3: Jam3/Javascript-Code-Conventions
JeopardyBot: kesne/jeopardy-bot
JSSolutions: JSSolutions/javascript
KickorStick: kickorstick/javascript
Kinetica Solutions: kinetica/javascript
Lonely Planet: lonelyplanet/javascript
M2GEN: M2GEN/javascript
Mighty Spring: mightyspring/javascript
MinnPost: MinnPost/javascript
MitocGroup: MitocGroup/javascript
ModCloth: modcloth/javascript
Money Advice Service: moneyadviceservice/javascript
Muber: muber/javascript
National Geographic: natgeo/javascript
Nimbl3: nimbl3/javascript
Nulogy: nulogy/javascript
Orange Hill Development: orangehill/javascript
Orion Health: orionhealth/javascript
OutBoxSoft: OutBoxSoft/javascript
Peerby: Peerby/javascript
Razorfish: razorfish/javascript-style-guide
reddit: reddit/styleguide/javascript
React: facebook.github.io/react/contributing/how-to-contribute.html#style-guide
REI: reidev/js-style-guide
Ripple: ripple/javascript-style-guide
SeekingAlpha: seekingalpha/javascript-style-guide
Shutterfly: shutterfly/javascript
Sourcetoad: sourcetoad/javascript
Springload: springload/javascript
StratoDem Analytics: stratodem/javascript
SteelKiwi Development: steelkiwi/javascript
StudentSphere: studentsphere/javascript
SwoopApp: swoopapp/javascript
SysGarage: sysgarage/javascript-style-guide
Syzygy Warsaw: syzygypl/javascript
Target: target/javascript
TheLadders: TheLadders/javascript
The Nerdery: thenerdery/javascript-standards
T4R Technology: T4R-Technology/javascript
VoxFeed: VoxFeed/javascript-style-guide
WeBox Studio: weboxstudio/javascript
Weggo: Weggo/javascript
Zillow: zillow/javascript
ZocDoc: ZocDoc/javascript

⬆ back to top

Translation

This style guide is also available in other languages:

Brazilian Portuguese: armoucar/javascript-style-guide
Bulgarian: borislavvv/javascript
Catalan: fpmweb/javascript-style-guide
Chinese (Simplified): sivan/javascript-style-guide
Chinese (Traditional): jigsawye/javascript
French: nmussy/javascript-style-guide
German: timofurrer/javascript-style-guide
Italian: sinkswim/javascript-style-guide
Japanese: mitsuruog/javascript-style-guide
Korean: tipjs/javascript-style-guide
Polish: mjurczyk/javascript
Russian: leonidlebedev/javascript-airbnb
Spanish: paolocarrasco/javascript-style-guide
Thai: lvarayut/javascript-style-guide
Ukrainian: ivanzusko/javascript
Vietnam: hngiang/javascript-style-guide

The JavaScript Style Guide Guide

Reference

Chat With Us About JavaScript

Find us on gitter.

Contributors

View Contributors

License

(The MIT License)

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the 'Software'), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED 'AS IS', WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

⬆ back to top

Amendments

We encourage you to fork this guide and change the rules to fit your team's style guide. Below, you may list some amendments to the style guide. This allows you to periodically update your style guide without having to deal with merge conflicts.

Name		Name	Last commit message	Last commit date
Latest commit History 2,760 Commits
css-in-javascript		css-in-javascript
linters		linters
packages		packages
react		react
.editorconfig		.editorconfig
.gitignore		.gitignore
.npmrc		.npmrc
.travis.yml		.travis.yml
LICENSE.md		LICENSE.md
README.md		README.md
package.json		package.json

License

xalexec/javascript

Folders and files

Latest commit

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Airbnb Javascript 编码规范() {

目录

类型 Types

引用 References

对象 Objects

数组 Arrays

解构赋值 Destructuring

字符串 Strings

函数 Functions

箭头函数 Arrow Functions

类 & 构造函数 Classes & Constructors

模块 Modules

迭代器和Generators Iterators and Generators

Properties

变量 Variables

提升 Hoisting

比较运算符 & 等号 Comparison Operators & Equality

块 Blocks

Control Statements

注释 Comments

空白 Whitespace

逗号 Commas

分号 Semicolons

类型转换 Type Casting & Coercion

Naming Conventions

Accessors

Events

jQuery

ECMAScript 5 Compatibility

ECMAScript 6+ (ES 2015+) Styles

Testing

Performance

Resources

In the Wild

Translation

The JavaScript Style Guide Guide

Chat With Us About JavaScript

Contributors

License

Amendments

};

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